Waste of car wrecks is particularly difficult to recycle because of the variety of plastic materials contained therein, such as hard plastic elements, rubber, foams or cable systems. Moreover, these elements may include macromolecular compounds, i.e. hydrocarbon polymers such as: polyethylene, polypropylene, polymers containing halogens, i.e. poly(vinyl chloride), polychloroprene polytetrafluoroethylene, oxygen-containing polymers such as poly(ethylene terephthalate), poly(methyl methacrylate), polyoxymethylene; polymers containing nitrogen from the group of polyurethanes, polyamides, or polymers containing sulphur, i.e. vulcanised rubbers.
There are known various methods of waste incineration, the most popular including pyrolysis and gasification. In these processes, waste is preheated by diaphragms via reactor walls, in which the waste is utilised. Waste is converted to a gaseous or oil form. However, these processes generate a considerable amount of by-products, such as soot, which are problematic for further recycling.
Waste can be utilized by a pyrolysis process. Pyrolysis involves thermal decomposition of organic matter in an anaerobic atmosphere or in reduced level of oxygen. High-temperature pyrolysis, in the reaction temperature range from 500° C. to 1600° C. (which is higher than in low-temperature pyrolysis), allows thermal decomposition of carbon polymers contained in waste to generate synthesis gas.
Another known process is gasification of waste, wherein pure oxygen is used in the last stage of the process, which enables to achieve reaction temperature, at which the waste residues can be entirely burned. In that process, air, oxygen and water vapour can be used to achieve partial oxidation. Gasification can be carried out in reactors of various types, such as rotary kilns, fluidised-bed boilers or boilers with moving or circulating bed.
The conventional pyrolysis methods, which involve heating the material through walls of a process chamber have a number of disadvantages that limit their scope of application. The temperature of the process chamber walls is the highest in the chamber and, which causes layers of soot, dust and tar to settle on the walls, which significantly reduces the transfer of heat to the material. The efficiency of energy transfer to the material is reduced; the duration of the process is prolonged and the pyrolysis process inside the chamber is uneven (the rate of degradation is different in different areas of the process chamber). In addition, there occur strong corrosive processes, which reduce the lifetime of equipment. Moreover, it is difficult to control the process temperature conditions; therefore, the quality of obtained gases is often low and varies during the process.
There are also known other methods of polymer waste thermal decomposition, including use of microwave energy.
A U.S. Pat. No. 5,084,140 discloses a method for destruction of macromolecular waste, wherein the waste (which is not itself susceptible to microwave heating and may be, for example, plastics or polychlorinated biphenyl) is mixed with pulverulent carbonaceous material, which comprises elemental carbon or is degradable by microwave irradiation to elemental carbon (such as waste tire material). The mix is subjected to microwave irradiation in an inert atmosphere to cause pyrolysis of the plastics. Pulverized elemental coal that forms fluidised bed is heated with microwaves to a temperature of at least 400° C. and pulverised plastics are introduced into the fluidised bed, which are subjected to pyrolysis in the fluidised bed by the transfer of heat to the powdered particles of utilised plastic material. Pyrolysis products are essentially gases and liquid hydrocarbon fractions that can be used as a source of energy, whereas solid fraction hydrocarbons formed as by-products are fed back to the pyrolysis process.
A US patent application US20020189928 discloses a process for microwave destruction of harmful agents and waste. The waste is subject to microwave impact in the presence of a catalyst, such as activated carbon, carbon soot, charcoal or metal carbides or/and water and gas, for example: air. In the case of use of active carbon as the catalyst, the process is carried out under conditions of temperature near the room temperature and atmospheric pressure. Waste can be disposed in a form of solid, liquid or gaseous waste.
A US patent application US20040054240 discloses a medical waste treatment unit, wherein organic material is subject to polymer degradation in an installation comprising three chambers. In the first chamber, the waste is weighed and the first chamber is mixed with oxygen. Then the organic material is introduced to the second chamber, where it is sterilised and depolymerised non-pyrolytically under microwave effect in an anaerobic atmosphere, whereas the gases produced in the process are purified in a scrubber. After that, the utilised material is cooled and milled in the third chamber to a form, in which it can be stored.
There is a need to provide an alternative method of processing plastic waste by pyrolysis using microwave energy.